1. Field of the Invention
The present invention relates to a two-way optical communication module for combining/splitting a transmission light and a reception light using a directional optical coupler.
2. Description of the Related Art
FIGS. 12-14 show a conventional two-way optical communication module disclosed in Japanese Patent Application Laid-Open No. Hei 5-289120. FIG. 12 is a diagram for showing an overall configuration, FIG. 13[1] is an expanded view for showing part of FIG. 12, FIG. 13[2] is a cross-sectional view for showing FIG. 13[1], and FIG. 14 is a graph for showing wavelength characteristics of a directional optical coupler. The following will describe the module with reference to these figures.
As shown in FIG. 12, on the surface of an optical wave-guide board 200 and near its one side are provided parallel an optical wave-guide 251 coupled optically to a light emitting element 210, an optical wave-guide 252 coupled optically to a light receiving element 220, and an optical wave-guide 254 coupled optically to a light receiving element 230. Near the other side of the optical wave-guide board 200 and on its surface, on the other hand, is provided a trunk optical wave-guide 250 coupled optically to an optical fiber 500. The trunk optical wave-guide 250 is divided into two branches, one of which provides an optical wave-guide 253 and the other of which provides an optical wave-guide 254.
The optical wave-guide 253 is further divided into two branches, one of which provides an optical wave-guide 251 and the other of which provides an optical wave-guide 252. At a Y-form junction of the trunk optical wave-guide 250 and the optical wave-guides 253 and 254 is provided a beam combiner/splitter (wave-guide type directional optical coupler or Mach-Zehnder type combiner/splitter) 220, which splits a light coming in through the optical fiber 500 into two components of a short-wavelength band xcex11 and a long-wavelength band xcex12 to transmit them through the optical wave-guides 253 and 254 respectively.
Further, at a Y-form junction of the optical wave-guides 251, 253, and 253 is provided a directional optical coupler 400 which gives a larger coupling loss in the long wavelength band xcex12 to the optical wave-guide 252 than the others, thus preventing a leakage light of the long wavelength band xcex12 going through the optical wave-guide 253 from coming into the light receiving element 220.
Further, the optical wave-guide 251 has the light emitting element 210 disposed at its injection end (that is, an end surface of the optical wave-guide board 200), while the optical wave-guide 252 has, as opposed thereto, a light receiving surface of the light receiving element 220 disposed at its emission end (that is, the end surface of the optical wave-guide board 200). Also, the optical wave-guide 254 has, as opposed thereto, a light receiving surface of the light receiving element 230 disposed at its emission end (that is, the end surface of the optical wave-guide board 200).
The following will detail the directional optical coupler 400 with reference to FIGS. 13 and 14.
As shown in FIG. 13, an interconnection of the optical wave-guides 253 and 251 is bent in a reverse trapezoid shape to provide a core line 410 and a light-incident end of the optical wave-guide 252 is bent in a trapezoid shape to provide a core line 420 near and parallel to the core line 410, thus implementing the directional optical coupler 400.
In one example of a configuration of the core lines 410 and 420, a width b is 6 [xcexcm] and a height a is 6 [xcexcm] to form a rectangle, a length L is 1.81 [xcexcm], and a distance d between the core lines 410 and 420 is 3.6 [xcexcm]. Also, the refractive index of the core lines 410 and 420 is 1.468 and that of a clad 450 is 1.457.
FIG. 14 shows a wavelength characteristic of the directional optical coupler 400. A dotted line P-1 in FIG. 14 indicates a relationship of a coupling loss and a wavelength between the optical wave-guides path 253 and 251, while a solid line P-2 indicates that between the optical wave-guides 253 and 252. Since the directional optical coupler 400 has such wavelength characteristics as shown in FIG. 14, by selecting a wavelength band centering around a value of 1.31 [xcexcm] as the short-wavelength band xcex11 and a wavelength band centering around a value of 1.55 [xcexcm] as the long-wavelength band xcex12, it is possible to prevent a light of the long-wavelength band xcex12 from entering to the optical wave-guide 252, that is, the light receiving element 220.
In a conventional two-way optical communication module, a light of the short-wavelength band xcex11 and that of the long-wavelength band xcex12 emitted from the optical fiber 500 pass through the trunk optical wave-guide 250 and then enter the beam combiner/splitter 220 to be split in wavelength into the short-wavelength and long-wavelength bands xcex11 and xcex12. As a result, the light of the long-wavelength band xcex12 goes along the optical wave-guide 254 and enters the light receiving element 230. The light of the short-wavelength band xcex11 and a light of the long-wavelength band xcex12 which has leaked from the beam combiner/splitter 220, on the other hand, go along the optical wave-guide 253 and pass through the directional optical coupler 400 so that only the light of the short-wavelength band xcex11 may go along the optical wave-guide 252 to enter the light receiving element 220. In this case, the light of the short-wavelength band xcex11 originated from the light emitting element 210 passes through the optical wave-guide 251, the directional optical coupler 400, the optical wave-guide 253, the beam combiner/splitter 220, and the trunk optical wave-guide 250 and then enters the optical fiber 500.
This conventional two-way optical communication module, however, permits a light of the short-wavelength band xcex11 emitted from the light emitting element 210 to be divided and radiated when passing through the directional optical coupler but does not take into account how to control the lights thus divided and radiated. That is, in the detailed drawings of the directional optical coupler 400 shown in FIG. 13, one half of the light of the short-wavelength band xcex11 issued from the optical wave-guide 251 is guided to the optical wave-guide 253, whereas the other half of the light is transferred in power to the core line 420 to be radiated from the cut end of the optical wave-guide 252. Thus radiated light is reflected irregularly in the two-way optical communication module to provide a stray light, which then enters the light receiving element 220 for the short-wavelength band xcex11, thus deteriorating the reception sensitivity characteristics.
As shown in FIG. 13[1], the directional optical coupler 400 has such a construction that the distance between the two close core lines 410 and 420 made of the optical wave-guides becomes larger toward the cut end of the optical wave-guide 252. In this construction, therefore, the optical wave-guides are elongated, thus giving rise to a disadvantage of a difficulty to make compact the optical wave-guide board 200.
In view of the above, it is an object of the present invention to provide two-way optical communication module with improved reception sensitivity characteristics.
In order to achieve above mentioned object, a two-way optical communication module according to present invention comprising: a light emitting element for emitting a light with a first wavelength; a first optical wave-guide including, a curved wave-guide portion coupled to said light emitting element, and a straight wave-guide portion coupled to a core of an optical fiber; a light receiving element; a second optical wave-guide including, a curved wave-guide portion coupled to said light receiving element, and a straight wave-guide portion coupled to a clad of said optical fiber; and a directional optical coupler including said straight wave-guide portion of said first optical wave-guide and said straight wave-guide portion of said second optical wave-guide, for guiding a light with said first wavelength from said first optical wave-guide to said core of said optical fiber and a light with a second wavelength from said core of said optical fiber to said second optical wave-guide.
Here, said first wavelength and said second wavelength may be equal to each other. Further, said clad of said optical fiber and said straight wave-guide portion of said second optical wave-guide are coupled to each other with a gap provided therebetween, a value of said gap being designed so that a diameter of a spot of light on said optical fiber which is radiated from said straight wave-guide portion may be smaller than a fiber diameter of said optical fiber. Moreover, said directional optical coupler is replaced by a Mach-Zehnder type combiner/splitter, a wave-guide portion of said Mach-Zehnder type combiner/splitter replacing the straight wave-guide portion of said first and second optical wave-guide. Further more, an end of said straight wave-guide portion of said second optical wave-guide is bent externally in a radial direction or in a circumferential direction of said optical fiber.
In other word, above-mentioned two-way optical communication module has a configuration so that the leakage light appeared in the directional optical coupler is induced outside of the two-way optical communication module by inducing the leakage light to the clad of the optical fiber. By inducing the leakage light of the directional optical coupler to outside of the two-way optical communication module, thus, it is possible to suppress a stray light reflected irregularly in the two-way optical communication module. Moreover, at the directional optical coupler, two optical wave-guides can be introduced to the clad without large gap each other, thus the curved wave-guide is not required. As a result, a small optical wave-guide can be realized.
Moreover, a two-way optical communication module having an optical wave-guide board, said optical wave-guide board comprising thereon: a light emitting element for emitting a light with a first wavelength; a first optical wave-guide having one end thereof coupled to said light emitting element and the other end thereof coupled to a core of an optical fiber; a light receiving element; a second optical wave-guide having one end thereof coupled to said light receiving element; and a directional optical coupler which is comprised of part of said first optical wave-guide and part of said second optical wave-guide and which guides the light with said first wavelength from said first optical wave-guide to said core of said optical fiber and the light with a second wavelength from said core of said optical fiber to said second optical wave-guide. And on a surface of said optical wave-guide board is formed one trench perpendicular to an optical axis at said one end of said second optical wave-guide. This trench is comprised of a first wall surface through which the light with said second wavelength is emitted from said one end of said second optical wave-guide and a second wall surface by which the light with said second wavelength emitted from said first wall surface is reflected toward said light receiving element.
On said first wall surface is formed a first reflection film except part thereof where the light with said second wavelength is emitted. The stray light pass through the optical wave-guide board to the light receiving element is shut by the first reflection film.
Also, on said second end surface is formed a second reflection film at part thereof where the light with said second wavelength is reflected. The light with second wavelength emitted from the first wall surface is reflected on the second reflection film, thus, the light is induced to the light receiving element effectively.
Moreover, a method for manufacturing the two-way optical communication module according present invention, comprising the steps of: forming said trench in the surface of said optical wave-guide board; forming a metal film throughout said first and second wall surfaces; and removing said metal film except part thereof, thus forming said first and second reflection films. The first and second reflection film are formed by same metal film forming step and same metal film removing step at the same time.
Here, an electrode may be formed at same time to form the first and second reflection film. This electrode may be used for said light emitting element and said light receiving element.